1. Field of the Invention
This invention relates to polyamide-imides and is particularly concerned with mixed aromatic-aliphatic copolyamide-imides.
2. Description of the Prior Art
Copolyamide-imides which contain the residues from trimellitic anhydride and aliphatic dicarboxylic acids in the polymer chains along with an arylene radical are well known in the art. U.S. Pat. No. 3,300,420 discloses organic solvent insoluble polyamide-imide reaction products from trimellitic anhydride and aryl polyisocyanates including products wherein up to about 50 mole percent of the trimellitic anhydride may be replaced by an aromatic or aliphatic dicarboxylic acid. Such products are not melt processable. U.S. Pat. No. 3,803,100 describes polyamide-imides derived from trimellitic anhydride and arylene diisocyanates that are solvent soluble but infusible and up to 60 mole percent of the trimellitic anhydride may be replaced by an aromatic dicarboxylic acid. U.S. Pat. No. 3,817,926 describes organic solvent soluble polyamide-imides in which the trimellitic anhydride constituent may be replaced by up to 75 mole percent of a dicarboxylic acid, inclusive of aliphatic dicarboxylic acids, and this mixture reacted with an organic diamine first, followed by a diisocyanate. The polymer obtained thereby is characterized by a low inherent viscosity, that is to say, low molecular weight which does not give rise to a polymer having good physical properties. Similarly, U.S. Pat. No. 3,884,880 discloses trimellitic anhydride-aromatic diisocyanate based copolyamide-imides in which up to 50 mole percent of the anhydride is replaced by an aliphatic dicarboxylic acid wherein the resulting soluble polymers are of low inherent viscosity. U.S. Pat. No. 3,929,691 discloses mixed copolyamide-imides containing the trimellitimide-amide and aromatic amide residues which are solvent soluble and spinnable but which are not melt processable.
U.S. Pat. No. 3,937,673 describes the preparation of solutions of polyamide-imide resins by reaction of trimellitic anhydride, an aromatic diisocyanate and an aliphatic dicarboxylic acid having more than 6 carbon atoms. U.S. Pat. No. 4,115,372 describes injection moldable copolyamide-imides derived by reaction of an aromatic diisocyanate with trimellitic anhydride and an aliphatic dicarboxylic acid having 7 to 12 carbon atoms in the chain separating the carboxylic acid groups. The proportions of the anhydride and the dicarboxylic acid are such that the recurring units derived from the latter comprise 30 to 70 percent of the total recurring units. These copolyamide-imides are characterized by relatively high heat resistance [Heat deflection temperature (HDT) at 264 psi, as measured by ASTM-D648, of the order of 150.degree. C.] as well as by high impact strength and good melt processability.
We have now found that polyamide-imides, having improved properties compared with those of the above '372 patent, can be prepared from trimellitic acid, an aromatic dicarboxylic acid, an aliphatic dicarboxylic acid and an aromatic diisocyanate as will be described more fully hereafter.